When water depth becomes too great to economically allow the installation of fixed production platforms the wellhead trees are mounted subsea on the ocean floor. Due to the inaccessibility of the subsea tree, the producing well must be "killed" and the tree remotely retrieved and brought to the surface of the water for any maintenance that might be required. Due to the time and expense involved in "killing" the well and retrieving the tree, redundant pressure containing valves have been used or where possible the valves have been installed at the surface. Due to their potential for high wear rates, the adjustable chokes have previously been installed above the water surface. This has required a high pressure production flowline for each producing zone of each well in a given oil or gas field which would be routed from the subsea tree, along the ocean floor, to a gathering ship or platform. Also, since the various wells in a given field may not all have the same pressure their production cannot be comingled into one flowline and thereby use one surface located choke. Subsea chokes have been suggested for use in riser pressure equalization systems for controlling downhole pressure during drilling (U.S. Pat. No. 4,210,208) and to circulate out formation influx of a formation kick without having the choke on the drill ship (U.S. Pat. No. 4,046,191).
If the pressure reducing adjustable chokes could be installed directly onto the subsea trees then each well's reduced pressure production could be directed into a single subsea gathering line with it being necessary to route only this one single gathering line along the ocean floor instead of multiple high pressure flow lines.